Comments on Coupling Graphical and Regression Analyses of Ligand Effect Data

Quantitation of the ligand effect in oxo-transfer reactions of dioxo-Mo(VI) trispyrazolyl borate complexes

The oxygen atom transfer reactivity (OAT) of dioxo-Mo(VI) complexes of hydrotrispyrazolyl borate (hydrotris(3,5-dimethylpyrazolyl)borate, Tp(Me2); hydrotris(3-isopropylpyrazol-1-yl)borate, Tp(iPr)) with tertiary phosphines (PMe(3), PMe(2)Ph, PEt(3), PEt(2)Ph, PBu(n)(3), PMePh(2), or PEtPh(2)) has been investigated. In acetonitrile, these reactions proceed via the formation of a phosphoryl intermediate complex that undergoes a solvolysis reaction. We report the synthesis and characterization of several phosphoryl complexes. The rates of formation of phosphoryl complexes and their solvation were determined by spectrophotometry. The rates of the reactions and the properties of the phosphoryl species were investigated using the Quantitative Analysis of Ligand Effect (QALE) methodology. The results show that, at least in this system, the first step of the reaction is controlled primarily by the steric factor, and in the second step, both electronic and steric factors are important. We also analyzed the effect of ligands on the reaction rate i.e., Tp(Me2)vs. Tp(iPr).

Structural Properties and Dissociative Fluxional Motion of 2,9-Dimethyl-1,10-Phenanthroline in Platinum(II) Complexes

A dynamic 1H NMR study has been carried out on the fluxional motion of the symmetric chelating ligand 2,9-dimethyl-1,10-phenanthroline (Me2-phen) between nonequivalent exchanging sites in a variety of square-planar complexes of the type [Pt(Me)(Me2-phen)(PR3)]BArf, 1-14, (BArf = B[3,5-(CF3)2C6H3]4). In these compounds, the P-donor ligands PR3 encompass a wide range of steric and electronic characteristics [PR3 = P(4-XC6H4)3, X = H 1, F, 2, Cl 3, CF3 4, MeO 5, Me 6; PR3 = PMe(C6H5)2 7, PMe2(C6H5) 8, PMe3 9, PEt3 10, P(i-Pr)3 11, PCy(C6H5)2 12, PCy2(C6H5) 13, PCy3 14]. All complexes have been synthesized and fully characterized through elemental analysis, 1H and 31P{1H} NMR. X-ray crystal structures are reported for the compounds 8, 11, 14, and for [Pt(Me)(phen)(P(C6H5)3)]PF6 (15), all but the last showing loss of planarity and a significant rotation of the Me2-phen moiety around the N1-N2 vector. Steric congestion brought about by the P-donor ligands is responsible for tetrahedral distortion of the coordination plane and significant lengthening of the Pt-N2 (cis to phosphane) bond distances. Application of standard quantitative analysis of ligand effects (QALE) methodology enabled a quantitative separation of steric and electronic contributions of P-donor ligands to the values of the platinum-phosphorus 1J(PtP) coupling constants and of the free activation energies DeltaG++ of the fluxional motion of Me2-phen in 1-14. The steric profiles for both 1J(PtP) and DeltaG++ show the onset of steric thresholds (at cone angle values of 150 degrees and 148 degrees , respectively), that are associated with an overload of steric congestion already evidenced by the crystal structures of 11 and 14. The sharp increase of the fluxional rate of Me2-phen can be assumed as a perceptive kinetic tool for revealing ground-state destabilization produced by the P-donor ligands. The mechanism involves initial breaking of a metal-nitrogen bond, fast interconversion between two 14-electron three-coordinate T-shaped intermediates containing eta1-coordinated Me2-phen, and final ring closure. By use of the results from QALE regression analysis, a free-energy surface has been constructed that represents the way in which any single P-donor ligand can affect the energy of the transition state in the absence of aryl or pi-acidity effects.

Electronic and steric effects in fragmentation reactions of Os3(CO)9(μ-C4Ph4)

P-Donor nucleophiles of cone angle >or=145 degrees are known to react with Os3(CO)9(mu-C4Ph4) quite differently from those with cone angles <or=143 degrees. A detailed and systematic kinetic study of the rather slow kinetics of reactions with 11 of the larger nucleophiles is described, and the results are analysed according to standard QALE protocols. (QALE=quantitative analysis of ligand effects). The results are compared with those previously reported for 17 of the smaller phosphines, which react much more rapidly. The pronounced difference in behaviour between these two groups of nucleophiles is ascribable to major differences between the ease of formation of different intermediate Os3(CO)9L(mu-C4Ph4) adducts. The cluster is proposed to be capable of opening up to form a nido structure that has a small, well defined opening capable of accommodating any of the smaller nucleophiles quite easily. However, this opened nido structure is concluded to be extremely rigid and incapable of further opening to accommodate any of the larger nucleophiles. These have to take advantage of an alternative nido structure that contains a much larger opening but that requires considerably more energy for it to be formed. These results are compared with those for the clusters M5C(CO)15(M=Ru or Fe), which show somewhat similar behaviour. This is ascribed to the complexity of the structures of the clusters that allows them to form alternative nido structures of the sort described, something that more symmetrical clusters are apparently incapable of doing.

σ→σ* Transitions in Mn2(CO)8L2 complexes (L = P-donor ligands): π-acidity and oxygen pendant group effects in phosphite ligands

Comparison of the energies assigned to the sigma-->sigma* transitions in bis-axial Mn2(CO)8L2 complexes when L = phosphites and phosphinites with those when L = alkyl and aryl phosphines shows that the oxygen-containing ligands do show effects associated with their pi-acidity. However, the data also strongly suggest that the energies are affected by the presence of the oxygen atoms themselves to an extent that increases linearly with the number of oxygen atoms connected to the P-donor atoms. This "oxygen effect" is analogous to, but distinct from, the aryl effect that is also simply proportional to the number of aryl groups attached to the P-donor atoms. This resolves the incongruity of the claim that phosphites also show an "aryl effect", and suggests that specific "pendant group effects" due to the nature and number of atoms attached to the phosphorus atoms of P-donor ligands (aryl, oxygen, nitrogen, sulfur etc.) may be quite general.

Comments on the Electrostatic Minimum Potential (Vmin) as a Measure of the Electron-Donating Power of PZ3

In a recent publication, Suresh and Koga suggested that the molecular electrostatic potential minimum (V(min)) could be used as a quantitative measure of the "sigma donating power" of substituted phosphorus(III) ligands. In support of this assertion, they showed linear correlations between a number of physicochemical properties and V(min). Our examination of the analyses of the sets of experimental data used to support this assertion shows unequivocally that the variations in the properties cannot be described by a single parameter. Therefore, the utility of V(min) as a measure of the "sigma donating power" of substituted phosphorus(III) ligands remains unsubstantiated.

Gold(I) heteroatom-substituted imido complexes. Amino nitrene loss from

The heteroatom-substituted imido complexes [(LAu)3(mu-NX)]+ (X = NR2, R = Ph, Me, Bz; X = OH, Cl; L = a phosphine) have been prepared from the reactions of NH2X with [(LAu)3(mu-O)]+. Thermally unstable [(LAu)3(mu-NNMe2)]+ (L = P(p-XC6H4)3, X = H, F, Me, Cl, MeO) decompose to the gold cluster [LAu]6(2+) and tetramethyltetrazene Me2NN=NNMe2. The decomposition is first-order overall with a rate constant that increases with increasing pKa of the phosphine ligand. Activation parameters for the decomposition are deltaH(not equal to) = 99(4) kJ/mol and deltaS(not equal to) = 18.5(5) J/K.mol for L = PPh3 and deltaH(not equal to) = 78(3) kJ/mol and deltaS(not equal to) = -47(2) J/K.mol for L = P(p-MeOC6H4)3. The decomposition of analogous [(LAu)3(mu-NNBz2)]+ produces bibenzyl, indicative of the release of free amino nitrene Bz2NN.

Structure-Reactivity Correlations for the Dissociative Uncatalyzed Isomerization of Monoalkylbis(phosphine)platinum(II) Solvento Complexes

Complexes of the type cis-[PtL(2)Me(2)] (1-14) (L = an extended series of phosphines of widely different steric and electronic properties) were synthesized, characterized, and used as precursors for the formation of cis-monoalkylplatinum(II) solvento species in methanol. The cleavage of the first platinum-alkyl bond by protonolysis is a fast process, but the subsequent cis to trans isomerization of the cationic solvento species [PtL(2)(Me)(MeOH)](+) is relatively slow and it can be( )()monitored using (31)P NMR or conventional spectrophotometry. A large collection of (1)H and (31)P NMR data for cis-[PtL(2)Me(2)], cis-[PtL(2)Me(MeOH)](+), and trans-[PtL(2)Me(MeOH)](+) complexes showed interesting dependencies upon the size, the sigma-donor capacity, and the mutual position of the phosphines in the coordination sphere of the metal. The rate constants for isomerization of cis-[PtL(2)Me(MeOH)](+) were resolved quantitatively into steric and electronic contributions of the phosphine ligands, by means of correlations with parameters which reflect their sigma-donor ability (chi values) and steric requirements (Tolman's cone angles, theta). The electronic and steric profiles obtained for these reactions are discussed within the framework of a mechanism which involves dissociative loss of the solvent molecule and interconversion of two geometrically distinct 3-coordinate T-shaped 14-electron intermediates. The factors controlling the stability of these coordinatively unsaturated species are discussed. The electronic and steric influences of phosphines as "spectator" ligands in a dissociative process are compared with those shown by these ligands when used as nucleophiles in associative substitution processes. The activation parameters DeltaH() and DeltaS() were measured using both conventional isothermal and non-isothermal spectrophotometric kinetics.